Vacuum heat treatment technology

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Vacuum heat treatment technology will become a new solution for high-end mold manufacturing – SIMUWU Vacuum Equipments

1.Introduction
Vacuum heat treatment is a new type of heat treatment technology that combines vacuum technology and heat treatment technology. The vacuum environment in which vacuum heat treatment is located refers to an atmosphere environment below one atmospheric pressure, including low vacuum, medium vacuum, high vacuum and ultra-high vacuum. Vacuum heat treatment is practical It also belongs to atmosphere controlled heat treatment. Vacuum heat treatment is carried out under vacuum for all and part of the heat treatment process. Vacuum heat treatment can realize almost all heat treatment processes involved in conventional heat treatment, and the quality of heat treatment is greatly improved. Compared with conventional heat treatment, vacuum heat treatment can achieve no oxidation, no decarburization, no carburization, can remove phosphorous chips on the surface of the workpiece, and has the functions of degreasing and degassing, so as to achieve the effect of surface bright purification.

2.The performance characteristics and key technologies of mold vacuum heat treatment
Vacuum heat treatment refers to the technology in which metal parts are heated under vacuum or first vacuumed and then passed through inert gas, and then quenched and cooled in oil or gas. Since the workpiece is basically not oxidized during the vacuum heat treatment, and the steel part is not decarburized, the ventilating convection heating method can also make the heating uniform, reduce the temperature difference between the surface and the core, thereby also reducing distortion. At present, it has become an indispensable and preferred heat treatment process for high-quality molds. What’s more valuable is that vacuum heat treatment does not produce any environmentally polluting substances during the production process, so it is recognized as a clean production technology. It is no exaggeration to say that vacuum heat treatment has become the main symbol of the current advanced heat treatment production technology. The main characteristics of the current development of vacuum heat treatment are convection heating, high pressure gas quenching, low pressure carburizing, ion carburizing, acetylene carburizing and other aspects.

Vacuum heat treatment technology is a new type of heat treatment technology developed in recent years. It has the characteristics that are urgently needed in mold manufacturing, such as preventing heating oxidation and non-decarburization, vacuum degassing or degassing, and eliminating Hydrogen embrittlement, good surface quality, small deformation, excellent comprehensive mechanical properties, pollution-free, pollution-free, high degree of automation and a series of outstanding advantages, thereby improving the plasticity, toughness and fatigue strength of materials (parts). Vacuum heat treatment technology has developed rapidly in my country in recent years. It has the characteristics that are urgently needed in mold manufacturing.

After the parts are subjected to vacuum heat treatment, the distortion is small, the quality is high, and the process itself is flexible and pollution-free. Therefore, vacuum heat treatment is not only a necessary means for heat treatment of some special alloys, but also used in the heat treatment of general engineering steels, especially tools, molds and precision coupling parts. After vacuum heat treatment, the service life is longer than that of general heat treatment. Improvement. For example, after vacuum heat treatment, some molds have a lifespan that is 40 to 400% higher than the original salt bath treatment, while the life of many tools can be increased by about 3 to 4 times. In addition, the vacuum heating furnace can work at a higher temperature, and the workpiece can maintain a clean surface, which can accelerate the adsorption and reaction process of chemical heat treatment. Therefore, certain chemical heat treatments, such as carburizing, nitriding, chromizing, boronizing, and multiple co-infiltration can get faster and better results.

Factors such as slow vacuum heating and small temperature difference between the inside and outside of the parts determine the small deformation of the parts caused by the vacuum heat treatment process. The successful development and application of computer simulation technology of the heat treatment process makes the intelligent heat treatment of the mold possible. Due to the small batch (even single piece), multi-variety characteristics of mold production, and the high requirements for heat treatment performance and the characteristics of not allowing waste products, intelligent heat treatment of molds becomes necessary. Foreign industrialized countries, such as the United States, Japan, etc., have also developed rapidly in terms of vacuum high pressure gas quenching, and the main target is also molds.

The development of my country’s vacuum heat treatment technology should meet the needs of industrial development and technological progress. According to my country’s national conditions, we should focus on the development of urgently needed advanced vacuum heat treatment technology. The main priority vacuum heat treatment technologies are as follows: low temperature convection heating and hierarchical austempering Functional vacuum high-pressure gas quenching heat treatment technology and intelligent control system; continuous vacuum heat treatment production line and intelligent control system; vacuum cleaning technology; vacuum nitriding technology; carbon black pollution and sharp corner over-seepage problems in vacuum carburizing; various typical molds Institutionalization of steel vacuum heat treatment process; high efficiency of vacuum heat treatment furnace.

3. Development and application of mold vacuum heat treatment
Generally speaking, a specific space filled with a pressure lower than atmospheric pressure is called a vacuum. If the heating, heat preservation and other processes of general heat treatment are carried out in a negative pressure atmosphere, it is vacuum heat treatment. Vacuum heat treatment technology has excellent comprehensive mechanical properties and a series of outstanding advantages, and has been a hot spot in the development of international heat treatment technology for many years. In thermal processing production, the combination of traditional heat treatment and modern vacuum technology, computer technology, and automation technology constitutes advanced vacuum heat treatment technology.

The main applications in the vacuum heat treatment of molds are vacuum oil quenching, vacuum quenching and vacuum tempering. In order to maintain the excellent characteristics of the vacuum heating of the workpiece (such as the mold), the selection and formulation of the coolant and the cooling process are very important. The mold quenching process mainly adopts oil cooling and air cooling. For mold working surfaces that are no longer machined after heat treatment, vacuum tempering should be used as much as possible after quenching, especially vacuum-quenched workpieces (molds), which can improve the mechanical properties related to surface quality, such as fatigue performance, surface brightness, Corrosion resistance, etc. The research and application of vacuum heat treatment process has covered many fields such as annealing, oil (gas) quenching, high pressure gas quenching, carburizing, nitriding, metalizing, tempering, sintering, brazing, coating, cleaning and so on. Great progress.

After vacuum heat treatment, the die steel has a good surface condition and small deformation. Compared with quenching in the atmosphere, the surface hardening of the mold after vacuum oil quenching is more uniform and slightly higher. The main reason is that the surface of the mold steel is in an active state during vacuum heating, without decarburization, and no oxide film that hinders cooling. When heated under vacuum, the surface of the steel has a degassing effect, so it has higher mechanical properties. The higher the vacuum in the furnace, the higher the bending strength. After vacuum quenching, the fracture toughness of the steel is improved, and the life of the die is generally increased by 40% to 400% or even higher than that of the conventional process. The vacuum quenching technology of cold work die has been widely used.

In the early stage of the development of vacuum heat treatment technology in my country in the 1970s, people mainly studied and discussed the basic properties of vacuum heat treatment, heating characteristics, metal evaporation problems, basic laws of metal heating processes under vacuum, and deformation problems. At the same time, research on typical heat treatment processes was carried out. The research and application of vacuum oil quenching and vacuum air quenching technology have laid a foundation for the research and application of vacuum heat treatment technology. Since the 1980s, vacuum heat treatment technology has developed rapidly in my country, and more advanced equipment has been introduced, and the application of vacuum heat treatment (the hardness test method of heat treatment workpieces in pit furnaces) has become increasingly widespread.

The research and application of high-pressure gas quenching technology, vacuum carburizing technology, vacuum sintering, vacuum brazing, and vacuum ion carburizing (nitrogen, metal) technology have been carried out one after another. Vacuum heat treatment technology has begun to move from research trials and small-scale production to industrial production. With the advancement of technology, since the 1990s, vacuum heat treatment technology has appeared many new technologies and features, the application of high-pressure gas quenching and ultra-high pressure gas quenching, vacuum heating hot air circulation tempering and rapid cooling technology, vacuum cleaning technology, Vacuum heat treatment equipment and process intelligent control system, vacuum nitriding technology, vacuum ion carburizing and other technologies are developing vigorously, and they have become hot spots for new technology development in developed countries such as the United States and Japan. my country has followed the international advanced technology and actively researched in accordance with the national conditions. It has developed new vacuum heat treatment technology and new equipment, such as high-pressure gas quenching process, vacuum tempering hot air circulation rapid cooling technology, continuous vacuum heat treatment process and vacuum carburizing and vacuum nitriding processes Wait. In the fields of vacuum cleaning technology, vacuum heat treatment process intelligent control system and high temperature ion heat treatment, research and exploration and industrial application development tests are also being carried out. The research and application of vacuum heat treatment technology in my country has spread to vacuum annealing, vacuum oil (gas) quenching, vacuum high pressure gas quenching, vacuum negative pressure and high flow rate gas quenching, vacuum carburizing, nitriding, vacuum tempering, vacuum sintering, vacuum brazing , Vacuum metalizing, vacuum ion carburizing, vacuum cleaning, vacuum spraying and other broad fields have made great progress. There are currently 33 professional vacuum heat treatment equipment manufacturers in my country. The main products are vacuum oil (gas) quenching furnaces, vacuum high pressure gas quenching furnaces, vacuum tempering furnaces, vacuum brazing furnaces, vacuum sintering furnaces, vacuum carburizing furnaces, and ion infiltration furnaces. Vacuum heat treatment equipment such as carbon (nitrogen) furnace and vacuum continuous furnace.

The vacuum heat treatment equipment for molds is a vacuum heat treatment furnace. The vacuum heat treatment furnace only appeared in the 1920s, and it has been more than half a century. In the 1960s, due to the wide application of graphite materials and the urgent needs of aerospace, electronics and other industries, the vacuum heat treatment furnace With rapid development, various types of vacuum furnaces have appeared, which have been improved and developed in terms of quantity, performance, and application, and are constantly developing in the direction of automation, large-scale and continuous development. At present, there are many manufacturers that produce vacuum furnaces and provide vacuum heat treatment. There are vacuum furnaces in the heat treatment workshops of larger American factories, and there are hundreds of factories that can accept vacuum heat treatment. Japan, Germany, the United Kingdom, France, Italy and other countries are also countries with more developed vacuum heat treatment furnaces, such as Japan’s Hayes Company and British Torvas Company. The development speed of vacuum heat treatment furnaces in China is even more impressive. Although the proportion is still very low, it has grown from nothing. In the past 20 years, there are more than 30 manufacturers that can produce vacuum heat treatment furnaces, such as the famous Shanghai Gehang Vacuum Technology Co., Ltd. The company is an expert in the field of vacuum heat treatment equipment manufacturing in China. It has produced nearly a thousand vacuum heat treatment furnaces. Various types of vacuum furnaces under the SIMUWU brand have been exported to more than 50 countries and regions.

With the development of my country’s automobile industry, the manufacturing level of automobile molds has also been improved day by day. In automobile production, almost all types of molds have been explored, such as cold die, hot forging die, cold heading die, hot heading die, die casting die and casting mold, etc. Therefore, the demand for molds in automobile manufacturing is particularly large. Its total output value has surpassed the machine tool industry in industrially developed countries. The improvement of automobile quality, the increase of production efficiency, the reduction of costs, and the speed of product upgrading depend to a large extent on the manufacturing precision and quality of the mold, the manufacturing cycle, the production cost and the service life. So some people say that mold is the foundation of the development of the automobile industry, and it is a sign of the maturity of the automobile manufacturing industry. The foundation supporting the development of the mold manufacturing industry is the mold material industry. In recent years, with the introduction and improvement of technology, the mold manufacturing industry has developed extremely rapidly.

4. Method of mold vacuum heat treatment
The characteristics of vacuum heat treatment are exactly what is urgently needed in mold manufacturing. Such as preventing heating oxidation and non-decarburization, vacuum degassing or degassing, eliminating hydrogen embrittlement, thereby improving the plasticity, toughness and fatigue strength of the material (mold). Factors such as slow vacuum heating and low temperature inside and outside the parts determine the small deformation of the parts caused by the vacuum heat treatment process. According to the different cooling media used, vacuum quenching can be divided into vacuum oil quenching, vacuum quenching, vacuum water cooling and vacuum nitrate austempering. Vacuum oil quenching, vacuum quenching and vacuum tempering are mainly used in vacuum heat treatment of molds. In order to maintain the excellent characteristics of mold vacuum heating, the selection and formulation of coolant and cooling process are very important. The mold quenching process mainly uses oil cooling and air cooling. For mold working surfaces that are no longer mechanically processed after heat treatment, vacuum tempering should be used as much as possible after quenching, especially vacuum quenched molds, which can improve the mechanical properties related to surface quality, such as fatigue performance, surface brightness, corrosion resistance, etc.

Because the heating and cooling of metal workpieces require dozens or even dozens of actions to complete. These actions are carried out in the vacuum heat treatment furnace and cannot be accessed by the operators. Therefore, the requirements for the degree of automation of the vacuum heat treatment electric furnace are relatively high. At the same time, some actions, such as the quenching process of the metal workpiece after heating and holding, require six or seven actions and must be completed within 15 seconds. To complete many actions under such agile conditions, it is easy to cause operator tension and constitute misoperation. Therefore, only high automation can accurately and timely coordinate according to procedures. The vacuum heat treatment of metal parts is carried out in a closed vacuum furnace, and strict vacuum sealing is well known. Therefore, obtaining and maintaining the original air leakage rate of the furnace and ensuring the working vacuum of the vacuum furnace are of great significance to ensuring the quality of the vacuum heat treatment of the parts. Therefore, a key problem of the vacuum heat treatment furnace is to have a reliable vacuum seal structure. In order to ensure the vacuum performance of the vacuum furnace, a basic principle must be followed in the structural design of the vacuum heat treatment furnace, that is, the furnace body should be airtightly welded, and at the same time, there should be as few or no holes on the furnace body as possible, and less or avoid dynamic sealing. Structure to minimize the chance of vacuum leakage. The parts and accessories installed on the vacuum furnace body, such as water-cooled electrodes and thermocouple lead-out devices, must also be designed with a sealed structure. Most heating and thermal insulation materials can only be used under vacuum.

The heating and thermal insulation linings of the vacuum heat treatment furnace work under vacuum and high temperature. Therefore, these materials have requirements for high temperature resistance, good radiation results, and low thermal conductivity. The anti-oxidation performance is not high. Therefore, the vacuum heat treatment furnace widely uses tantalum, tungsten, molybdenum and graphite as heating and heat insulation materials. These materials are very easy to oxidize in the atmosphere, therefore, ordinary heat treatment furnaces cannot use these heating and heat insulating materials. Adopt water-cooling device: the furnace shell, furnace cover, electric heating element, water-cooled electrode, intermediate vacuum insulation door and other components of the vacuum heat treatment furnace are all working under vacuum and heating. Working under such extremely unfavorable conditions, it is necessary to ensure that the structure of each component is not deformed or damaged, and that the vacuum sealing ring is not overheated or burned. Therefore, each component should be equipped with water-cooling devices according to different conditions to ensure that the vacuum heat treatment furnace can operate normally and have sufficient service life.

Using low voltage and high current: the energized conductor in the vacuum container will produce glow discharge under higher voltage. In the vacuum heat treatment furnace, severe arc discharge will burn down the electric heating elements, heat insulation layer, etc., causing major accidents and losses. Therefore, the operating voltage of the electric heating element of the vacuum heat treatment furnace generally does not exceed 80-100V. At the same time, effective measures should be taken when designing the structure of the electric heating element, such as avoiding sharp parts as much as possible, and the distance between the electrodes should not be too small to prevent the generation of glow discharge or arc discharge.

5. Commonly used mold vacuum quenching, tempering and nitriding process
Vacuum quenching technology has been widely used in various carburized structural steels, alloy steels, high-speed steels, stainless steels and various ageing alloys. After the mold is vacuum quenched, the surface brightness is good, the deformation is small, the hardness is high and uniform, and the service life is long.

Vacuum annealing technology is similar to before. Vacuum annealing of the mold can finally get a bright surface. This has obvious advantages for some molds with high surface quality requirements. Now the mold materials used in vacuum annealing include steel, copper and their alloys.

At present, vacuum high-pressure air-cooling quenching technology has developed rapidly, and negative pressure (< 1 × 10 5 Pa), high-flow rate air-cooled and pressurized (1 × 10 5～ 4 × 10 5 Pa) gas has appeared successively. New technologies such as cold, high pressure (5 × 10 5 ～ 1 0 × 10 5P a) air cooling, ultra-high pressure (10×105～20×105Pa) air cooling, etc., not only greatly improve the vacuum quenching capacity, After quenching, the surface brightness of the workpiece is good, the deformation is small, and it has the advantages of high efficiency, energy saving, and no pollution.

The purpose of vacuum high-pressure air quenching is the quenching and tempering of materials, the solid solution and aging of stainless steel and special alloys, ion carburizing and carbonitriding, as well as vacuum sintering, cooling and quenching after brazing.

When quenching with 6×105Pa high-pressure nitrogen, the load to be cooled can only be loose. High-speed steel (W6Mo5Cr4V2) can be hardened to 70-100mm, and high-alloy hot-work die steel (such as 4Cr5MoSiV) can reach 25-100mm. When cooling and quenching with 10×105Pa high-pressure nitrogen, the cooling load can be intensive, which is about 30%-40% higher than the load density when cooling with 6×105Pa. When cooling and quenching with 20×105Pa ultra-high pressure nitrogen or a mixture of helium and nitrogen, the cooling load is dense and can be bundled together. Its density is increased by 80%～150% compared with 6×105Pa nitrogen cooling. It can cool all high speed steel, high alloy steel, hot work die steel and Cr13% chromium steel and more alloy oil quenched steel, such as larger Size of 9Mn2V steel.

The cooling capacity of a dual-chamber air-cooled quenching furnace with a separate cooling chamber is better than that of the same type of single-chamber furnace. The cooling effect of a 2×105Pa nitrogen-cooled double-chamber furnace is equivalent to that of a 4×105Pa single-chamber furnace. However, operating costs and maintenance costs are low. The level of my country’s basic material industry (graphite, molybdenum, etc.) and supporting components (motors) needs to be improved. Therefore, while improving the quality of 6×105Pa single-chamber high-pressure vacuum protection, the development of double-chamber pressurized and high-pressure air-cooled quenching furnaces is more in line with my country’s national conditions.

Compared with conventional gas carburizing technology, low-pressure (vacuum) carburizing technology has a series of advantages, such as no internal oxidation and abnormal organization in the carburized layer. The surface of the workpiece is bright, the carburizing rate is fast, and it is easy to realize ion carburizing and high-temperature carburizing, and further improve the carburizing rate. High-pressure gas quenching is performed after low-pressure carburizing, which can minimize the distortion of the carburized and quenched steel parts. In recent years, the semi-continuous and continuous low-pressure carburizing furnaces and continuous carburizing and quenching return water continuous production lines have created good conditions for the promotion of low-pressure carburizing technology. Due to the special injection of carburizing gas into the furnace mechanism and the reduction of the gas pressure below 1×10-4MPa, as well as the implementation of pulse gas supply, the phenomenon of carbon black storage in the furnace and the surface of the workpiece no longer exists. The current low-pressure carburizing technology can ensure that the uniformity of the carburized layer is within 0.1mm when the depth of the carburized layer is 0.6mm.

When low-pressure carburizing, there is no equilibrium reaction between carburizing gas and steel parts. Steel parts are in hydrocarbon gas at high temperature, and the surface can reach a high carbon content within a few minutes, which increases the difficulty of process control . At present, the test data of carbon transfer rate under different temperature and pressure conditions are still used to control the production. These data are stored in databases and computer programs related to carbon infiltration and diffusion. When the calculated carbon content on the surface of the steel reaches the saturation limit of austenite, the control system stops carburizing. This process only lasts for a few minutes. Carburization, exhaust and diffusion are performed in sequence until the specified thickness of the carburized layer is obtained.

During the vacuum nitriding process, because other gases and adherents absorbed by the mold surface will be pumped out of the furnace during “pumping”, the activity of the steel surface is greatly improved, which promotes the effect of the active nitrogen in the nitriding process. The absorption of Yu speeds up the nitriding process. In addition, before the heating of ordinary gas nitriding, NH3 must be introduced to remove the non-nitriding atmosphere in the furnace, especially the oxidizing atmosphere, but there will still be a small amount of oxidizing atmosphere in the furnace, even if it is a high-purity gas, it also contains 0.1 Impurities. In a vacuum of 1.33Pa, the impurity content is only 0.001. It can be seen that the purification effect of vacuum nitriding has two aspects, one is to purify the surface of the workpiece, and the other is to purify the atmosphere in the furnace. In addition, compared with ion nitriding, vacuum nitriding has the following characteristics: First, the nitriding temperature is uniform and easy to control, while the workpiece is heated by the bombardment of positive ions during ion nitriding, and there are certain difficulties in temperature measurement and control. ; The second is the high quality of the nitriding surface. During the ion nitriding process, the positive ions continuously bombard the surface of the workpiece, resulting in many small pits on the surface, especially when the current is large, this phenomenon is more serious; the third is the uniform distribution of the nitriding layer of the workpiece. Especially for workpieces with complex shapes and small holes, it is difficult to obtain a uniform nitriding layer during ion nitriding. Vacuum nitriding is due to intermittent suction.

The atmosphere in the furnace flows more evenly than ordinary gas during nitriding, which ensures that all parts of the workpiece can get a uniform nitriding layer. The vacuum nitriding technology has not been developed for a long time, and further research is needed.

6. Heat treatment technology of mold vacuum furnace
Vacuum heat treatment furnace is an important equipment to realize vacuum heat treatment. The pressure in the furnace is lower than normal pressure. The atmosphere in the furnace is equivalent to a kind of gas, which can prevent the heat treatment from oxidation. The vacuum treatment furnace has a wide range of uses, mainly used for vacuum tempering, aging, annealing, and quenching of structural steel, stainless steel, heat-resistant alloys, precision alloys and materials. It can also be used for vacuum degassing, brazing and sintering, etc. aspect. The vacuum hot buried furnace is safe and reliable to use, can be purchased and operated automatically, and there is no pollution and other problems. However, the equipment is more complicated and costly. There are many types of vacuum heat treatment furnaces, which are generally classified according to the following special certificates: According to their use, they can be divided into: tempering furnace, annealing furnace, quenching furnace, brazing furnace and sintering furnace. According to the degree of vacuum, they can be divided into: low vacuum furnace, Medium vacuum furnace and high vacuum furnace can be divided into resistance heating, induction heating, electron beam heating, plasma heating and gas heating vacuum furnace according to the heat source.

The most important influence on the life of the mold is the design of the mold (including the correct selection of materials), the material of the mold, the heat treatment of the mold, and the use and maintenance of the mold. If the design of the mold is reasonable and the material is high-quality, the quality of the heat treatment directly determines the service life of the mold. At present, both at home and abroad are trying to use more advanced heat treatment methods to improve the performance of the mold and extend the service life of the mold. Vacuum heat treatment is one of the more advanced methods in mold heat treatment. From the perspective of mold heat treatment, the state of heat treatment processing equipment, heat treatment process, and production process control are particularly important. The advanced nature of equipment is a prerequisite to ensure the realization of advanced technology. Vacuum high pressure gas quenching furnace is the most ideal equipment for vacuum heat treatment.

The vacuum furnace has the effect of non-decarburization and non-oxidation, with uniform temperature, controllable heating and cooling speeds, and can realize different processes. Because there is no pollution, the vacuum furnace is internationally recognized as “green heat treatment”. Now there are 2-20bar vacuum high-pressure gas quenching furnaces in the world, which can fully meet the requirements of vacuum heat treatment of molds. According to the heating element inside and outside the vacuum vessel, the vacuum heat treatment furnace can be divided into an internal heating furnace and an external heating furnace. The external heating type vacuum heat treatment furnace has an ordinary air furnace outside the furnace pot of this vacuum furnace, and the workpiece is indirectly heated by heating the vacuum container. This kind of heat treatment furnace has poor thermal efficiency and long cooling time. In addition, due to the temperature difference between the workpiece and the furnace tank, the vacuum external pressure will also cause the furnace tank to deform and break. Although this kind of heat treatment furnace is cheap, it is not suitable for actual industrial production.

The internal heating type vacuum heat treatment furnace has an electric heater and an insulator in the vacuum container, and the container itself is water-cooled. When the processing temperature is low, only the vacuum sealed part is water-cooled or not. Compared with the external heating type, the internal heating type vacuum furnace has high thermal efficiency because the workpiece faces the heating element. Heating element materials include electric heating alloys, semi-metals, and zirconium heating elements.
In the process of mold heat treatment, the process parameters used also have a vital impact on mold performance: it includes heating temperature, heating rate, holding time, cooling method, cooling rate, etc. The correct heat treatment process parameters can ensure the best performance of the mold, otherwise, it will produce undesirable or even serious consequences. Practice has shown that the correct heat treatment process can obtain an excellent structure, and an excellent structure can ensure excellent mechanical properties. Appropriate processing methods can effectively control the deformation and cracking of the mold during heat treatment. In the process of mold heating and cooling, the difference between mold surface temperature and core temperature (non-uniformity of heating and non-uniformity of cooling) is the main factor causing mold deformation. (The vacuum furnace has the ability to control the heating rate and cooling rate). Different process methods can make the mold meet different conditions of use and different performance requirements.

From the perspective of the service life of the mold, meeting the hardness requirements is only one aspect of meeting the technical requirements of the mold. It also has some performance requirements that are not easy to measure, such as strength and toughness. The quality of the mold cannot be determined by the hardness index. It is impossible to use the hardness measurement method to finally test the service life of the mold. As a special process (ie special process), heat treatment can only pass process verification and performance experiment. , Confirm reasonable process parameters, and strictly implement the confirmed process parameters (process control) to ensure the reliability and stability of product quality. A large amount of data shows that the cold-punching dies processed by vacuum heat treatment have less deformation and rarely occur wire-cutting cracking and abrasion. The die-casting mold adopts advanced technology to reduce the cracking of the mold and the phenomenon of mold sticking in use to a certain extent. The vacuum high-pressure gas quenching process has the advantage of freely controlling the heating and cooling rates, and can program different process parameters to obtain the expected metallographic structure and properties. Vacuum heat treatment is a combination of modern vacuum technology and metal heat treatment. Mold vacuum heat treatment has the advantages of no environmental pollution, high quality, long life, low energy consumption and lower cost. The vacuum heat treatment of molds in industrially developed countries has reached more than 90%. my country started late, less than 10%. The promotion of application and popularization of vacuum heat treatment will contribute to my country’s cutting-edge technology, machinery industry and mold manufacturing.

7. Development trend of mold vacuum heat treatment

With the rapid development of the national economy, the market demand for molds continues to grow. In recent years, the mold industry has been developing rapidly at a growth rate of about 15%, and all the components of mold industry enterprises have also undergone tremendous changes. In addition to state-owned professional mold factories, collectives, joint ventures, sole proprietorships and private companies have also developed rapidly. With the accelerating pace of integration with international standards and increasing market competition, people have increasingly realized the importance of product quality, cost, and the ability to develop new products. Mold manufacturing is one of the most basic elements in the entire chain. Mold manufacturing technology has now become an important indicator to measure the level of a country’s manufacturing level, and to a large extent determines the living space of an enterprise. In recent years, many mold companies have increased their investment in technological progress and regard technological progress as an important driving force for enterprise development. Mold vacuum heat treatment has the following development trends:

Convection heating technology: Convection refers to the relative displacement between various parts of the fluid. It relies on the heat transfer method caused by the mixing and movement of cold and hot fluids. Convection is the main method of heat transfer in liquid or gas. Heating the workpiece under vacuum conditions mainly relies on radiation. Because the radiation heat transfer is proportional to the 4th power of the temperature, the radiation effect is not high below 850℃, and the heating speed of the workpiece is very slow; secondly, because some alloy elements in the metal material are heated under low pressure conditions, there is evaporation loss. , It will cause the lack of surface alloying elements, which will affect the performance of the surface layer after quenching. Convection heating technology means that the furnace chamber of the vacuum furnace is first pumped to a certain degree of vacuum, and then the inert (Ar, He), neutral N2 or reducing H2 gas of 0.1 ~ 0.2 MPa is passed through, and heating is performed under sufficient agitation. Compared with pure vacuum conditions, the heating speed can be at least doubled.

High-pressure gas quenching technology: After high-speed steel, high-carbon and high-alloy die steel, and tool dies are quenched in vacuum heating oil, a high-hardness white layer composed of alloy carbides will appear on the surface of the blade and thin-walled parts, which is very brittle. It is prone to chipping during service, causing early failure. The melting point of high-speed steel is low, and the austenitizing temperature of quenching heating is close to the melting point. When heated in a vacuum, the surface is purified to a large extent, so that the surface atoms maintain higher activity, while the oil under a certain vacuum When the medium quenching and cooling, the surface of the tool and die will also increase carbon at the moment of the oil and gas film, thereby further reducing the melting point, resulting in partial melting of the surface, so it is not feasible to quench the high carbon and high alloy tool and die steel in oil after vacuum heating. High-carbon, high-alloy tool and die steels are vacuum-heated and austenitized, and quenched and cooled in inert, neutral or reducing gas are inert. Quenching and cooling in neutral or reducing gas is the only feasible way to ensure quality. However, air cooling under negative pressure and normal pressure cannot achieve good hardening effect for large workpieces or batches of charge. On the one hand, the gas itself needs to be cooled, and on the other hand, to increase the cooling capacity of the gas under limited volume conditions, it can only be achieved by increasing the amount of cooling gas, that is, increasing the pressure of the cooling gas. Ipsen has developed a Flux-Sensor (air flow sensor), which can be placed in the charge to directly measure the cooling curve, measure the heat flow across the surface of the sensor, and calculate the heat transfer coefficient. This sensor can be placed in any vacuum protection and used under various charging conditions to obtain the cooling heat flow and heat transfer coefficient values ​​per heat. The main function of the sensor is to distinguish the quenching capacity of different vacuum furnaces, and secondly to verify the reproducibility of the quenching cycle of each furnace. Using this measuring device, coupled with an appropriate software system, push the newly measured heat transfer coefficient to other materials and parts of different sizes, and predict the quenching and cooling conditions.

Vacuum carburizing technology: Compared with conventional gas carburizing, low-pressure carburizing has a series of advantages, such as no internal oxidation and abnormal organization in the carburized layer, bright surface of the workpiece, fast carburizing, and easy to achieve ion carburizing and high temperature Carburizing to further increase the infiltration rate. High-pressure gas quenching is performed after low-pressure carburizing, which can minimize the distortion of the carburized and quenched steel parts. In recent years, the semi-continuous and continuous low-pressure sand carbon furnaces and continuous carburizing and quenching backwater continuous production lines have created good conditions for the promotion of low-pressure carburizing technology. It is conceivable that the technological innovation climax of a large number of carburized and quenched parts in the automotive industry to switch to low-pressure carburizing is just around the corner.

During low-pressure carburizing, because there is no equilibrium reaction between carburizing gas and steel parts, steel parts are in hydrocarbon gas at high temperature, and the surface can reach a high amount of carbon in a few minutes, thereby increasing the process control It is controlled by the test measurement data of the carbon transfer rate under the piece. These data are stored in databases and computer programs related to carbon infiltration and diffusion. When the calculated carbon content on the surface of the steel reaches the saturation limit of austenite, the control system stops carburizing (strong infiltration). This process only lasts for a few minutes. Carburization, exhaust and diffusion are performed in sequence until the specified thickness of the carburized layer is obtained.

Auxiliary materials and ancillary equipment: Quenching medium and other process materials Quenching medium is the most used and most widely used process material in production, which directly affects the performance and distortion of parts after quenching. At present, while supplying products, manufacturers also actively measure the performance of quenching medium, conduct process experiments and purchase cooling systems for users.

One of the development directions of heat treatment technology improvement is to combine traditional heat treatment technology with advanced surface treatment technology to improve the service life of die casting molds. Through the continuous development of die-casting die heat treatment technology, the die-casting die will obtain good performance while increasing the surface quality of the die-casting die, so that the domestic die-casting industry will develop well and quickly. Recently, relevant experts have developed substrate pretreatment technology for mold substrate and surface treatment technology. On the basis of traditional technology, suitable processing techniques for different mold materials have been proposed to improve mold performance and increase mold life.

In recent years, through the introduction of advanced technologies and the digestion and absorption of imported technologies, my country’s heat treatment production technology has undergone outstanding changes and has achieved considerable development. Due to the low starting point and weak foundation in the past, as a whole, there is still a big gap between the level of heat treatment production technology and advanced industrial countries. In the current general trend of world economic integration, improving the quality of mechanical products and competitiveness in domestic and foreign markets is an urgent matter. Heat treatment production technology must also catch up to seize opportunities and meet new challenges in order to reduce and shorten The gap in the international advanced level will enable my country’s heat treatment technology and mechanical product quality to enter the world’s advanced ranks as soon as possible. According to the forecast of the American heat treatment industry in 2020 by the American Institute of Metals Heat Treatment Society, the American Institute of Metal Processing, and the U.S. Department of Energy’s Industrial Technology Agency, the future heat treatment industry must have first-class quality and produce parts with zero change rate. In the entire process, zero dispersion is typical, the energy utilization rate is increased to 80%, the working environment is good, clean and pollution-free, the standard closed-loop control system is used in the production, and the intelligent system control determines the performance of the product. The result of the integrated technology makes the process Time is reduced by 50% and cost is reduced by 75%. All these ideas provide a broad stage and opportunities for the development of vacuum heat treatment technology. Vacuum heat treatment accounts for 15% of current heat treatment production in the United States, and this development trend will further increase in the future. SIMUWU vacuum furnaces sell well all over the world and contribute to high-end mold heat treatment!

8. Conclusion
Vacuum heat treatment technology has a series of outstanding advantages such as no oxidation, no decarburization, degassing, degreasing, good surface quality, small deformation, excellent comprehensive mechanical properties, no pollution, no pollution, and high degree of automation. It has always been the development of international heat treatment technology for many years. Hot spots, my country has also developed rapidly in recent years. Now, vacuum heat treatment has entered the continuous expansion of the technical field, the technological level is gradually improved, the vacuum heat treatment equipment is continuously improved and intelligent, and new technologies are emerging one after another in a stable development stage. The development of vacuum heat treatment technology is in the ascendant, and the prospect is bright.

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